The invention relates to an apparatus for detecting objects formed by labels on a carrier material, and sheets, particularly sheets of paper.
An apparatus of this type is known from German Patent DE-PS 195 21 129. This apparatus includes a capacitive sensor having two adjacent capacitor elements, which are spaced from one another opposite a capacitor plate. The carrier material with the labels is guided in the air gap between the capacitor plate and the capacitor elements.
For detecting the labels on the carrier material, a timing element that generates a pulse sequence and a low-pass filter are disposed downstream of each capacitor element. The outputs of the low-pass filters are guided to the inputs of a differential amplifier. This circuit can detect labels on the carrier material, even if the labels and the carrier material comprise transparent materials.
A disadvantage, however, is that such apparatuses are extremely sensitive to temperature influences, and humidity in particular. They also encounter difficulties in detecting metallized materials. Furthermore, the two capacitor elements can only detect the signal flanks occurring in the detection of a label edge. A prerequisite of this is that the labels and the carrier material be moved past the capacitor elements at a predetermined speed. Static detection with a stationary carrier material is not possible. A further drawback is that a reliable label detection is only possible if the label edges travel along a straight line transverse to the transport direction of the carrier material, so that an at least approximately constant spacing between adjacent labels must be maintained over the label width.